If the odds are dropping, that means that that people are betting heavily on one side of the bet or the other.
Dropping odds example
If a line opens at a certain price, and then injuries, suspensions, or some other factor impacts that potential game, the price on the game may drop.
For example, when Neymar was injured prior to the UEFA Champions League round of 16 tie between Manchester United and PSG, the line for Manchester United to advance moved down doom +300 to +200. When Manchester United lost the first leg 2-0 at home, the line to advance moved to PSG -10000.